Solvent recovery process



Jam-27, 1970l M. VAN TAssELL 3,492,222

SOLVENT RECOVERY PROCESS Filed Nov. 14. 1967 Relurn Benzene Nef C7*Aromaf/cs uwn/og auazuag Rell/rn Lean Solvent N VEN T0 R Harry M. VanTasse/l A TTOR/VE'YS United States Patent O 3,492,222 SOLVENT RECOVERYPROCESS Harry M. Van Tassell, Des Plaines, Ill., assiguor to UniversalOil Products Company, Des Plaines, Ill., a corporation of Delaware FiledNov. 14, 1967, Ser. No. 682,941 Int. Cl. C10g 21/28, 21/16 U.S. Cl.208-321 11 Claims ABSTRACT F THE DISCLOSURE Process for recovery ofspecified solvent from a nonaromatic raffinate which comprisesextracting thel solvent therefrom with a iirst aqueous stream to producea solvent free raffinate and a second aqueous stream containing solventand non-aromatic hydrocarbons. The second aqueous stream is thencontacted with aromatic hydrocarbons in a contacting zone underconditions which produce an aromatic stream containing non-aromatichydrocarbons and a third aqueous stream containing solvent and havingsubstantial freedom from non-aromatics. The process has particularyapplication in aromatic extraction processing wherein the specifiedsolvent comprises a sulfolane-type solvent, a polyalkylene glycol typesolvent, or other typically utilized water soluble solvents.

FIELD OF INVENTION The present invention relates to the solventextraction of aromatic hydrocarbons from a hydrocarbon charge stream.More particularly, the present invention relates to the recovery ofsolvents from a non-aromatic raflinate which is produced by the solventextraction of aromatics from a hydrocarbon charge stream. Morespecifically, the present invention relates to an improved process forthe recovery of solvent from the non-aromatic ratlinate by water Washingthe solvent-containing ratlinate and contacting the resultingsolvent-containing aqueous stream with aromatic hydrocarbon.

It is well known in the art that the raffinate which leaves theextraction zone of an aromatic hydrocarbon extraction process containssolvent. The solvent which is withdrawn in the raffinate stream must berecovered not only because it may interfere with subsequent rafnateprocessing or ultimate rainate use, but primarily because continual lossof solvent in the rainate stream is a prohibitive economic expense inthe aromatic extraction process. The recovery of the solvent from theraffinate stream may be typically accomplished by distillation of theraffinate or by a secondary solvent extraction process such as the waterwashing of the raiiinate.

A typical solvent which is utilized in commercial aromatics extractionand which may be recovered in accordance with the practice of thisinvention is a solvent of the sulfolane type. The solvent possesses afive-membered ring containing one atom of sulfur and four atoms ofcarbon with two oxygen atoms bonded to the sulfur atom of the ring.Generically, the sulfolane type solvents may be indicated as having thefollowing structural formula:

wherein R1, R2, R3, and R4 are independently selected from the groupcomprising a hydrogen atom, an alkyl group having from one to ten carbonatoms, an alkoxy radical having from one to eight carbon atoms, and anarylalkyl radical having from one to twelve carbon atoms.

3,492,222 Patented Jan. 27, 1970 Other solvents which may be includedwithin this process are the sulfolenes, such as 2-sulfolene or3-sulfolene which have the following structures:

O O S Cz \CH2 CH=CH Other typical solvents which have a high selectivityfor separating aromatics from non-aromatic hydrocarbons and which may beprocessed within the scope of the present invention are2-methylsulfolane, 2,4-dimethylsulfolane, methyl- 2 sulfonyl ether,N-aryl-3-sulfonylamine, 2-su1fonyl acetate, diethylene glycol, variouspolyethylene glycols, dipropylene glycol, various polypropylene glycols,dimethyl-sulfoxide, N-methyl pyrrolidone, etc.

The specifically preferred solvent chemical which is processed withinthe scope of the present invention is sulfolane having the followingstructural formula:

Cz \CH2 CH2- CH2 DESCRIPTION OF PRIOR ART The typical solventcomposition Within the scope of the present invention comprises amixture of water and one or more of the solvent chemicals herein noted.A particularly preferred solvent composition of the present inventioncomprises Water and sulfolane. In extracting aromatic hydrocarbons fromthe hydrocarbon mixture, it is known that the parains are the leastsoluble followed in increasing order of solubility by naphthenes,oleiins, diolens, acetylenes, sulfur containing hydrocarbons, nitrogencontaining hydrocarbons, oxygen containing hydrocarbons and aromatichydrocarbons. It is the practice to regulate the solubility of thehydrocarbons within the solvent composition by varying the water contentthereof. Thus, by adding more Water to the solvent, the solubility ofall components in the hydrocarbon mixture is decreased but thesolubility difference between components (selectivity) is increased.'Ihe net effect is to decrease the number of contacting stages requiredto achieve a given purity of aromatic extract, or to increase theresulting purity of the aromatic extract when the number of contactingstages is held constant.

The presence of water in the solvent composition provides a furtherprocessing benet in that it introduces a relatively volatile materialinto the fractionation system wherein the aromatic extract is separatedfrom the rich solvent composition. The water of the solvent compositionis vaporized at least in part to provide assistance in stripping alltraces of non-aromatic hydrocarbons out of the aromatic-rich solvent,and to provide assistance in stripping substantially all of the aromaticextract out of the final lean solvent.

It is, therefore, the practice to provide that the solvent compositioncontain from about 0.1% to about 20% by wt. of water. When the solventcomposition comprises chemical sulfolane, it is preferable that thesolvent cornposition contain from about 0.1% to about 1.0% of Water,while a solvent composition comprising a polyalkylene glycol preferablycontains from about 6% to 15% of water.

In the aromatic extraction process which is typical of the presentinvention, the aromatic-containing hydrocarbon feed stock is passed toan extraction zone which may comprise a tower containing suitablepacking such as Berl saddles or Raschig rings, or a tower containingsuitable trays, or a rotating disc contactor (RDC). The feed stock iscontacted therein with a lean solvent composition under conditionssufficient to produce a non-aromatic raflinate and an aromatic richsolvent. The rich solvent composition leaving the extraction zone ispassed to a rich-solvent separation zone which typically comprises oneor more fractionation columns which are operated in a manner sufcient toremove non-aromatics from the rich solvent and to recover a high purityaromatic extract and the final lean solvent composition. Thenon-aromatic fraction which is removed is normally returned to theextraction zone to provide a non-aromatic reux. Because the solventchemicals typically utilized are chemically unstable at elevatedtemperatures, the aromatic extract is normally removed from the solventcomposition with the assistance of steam sripping under operatingconditions which minimize thermal decomposition of the solvent chemical.The aromatic extract is separated from the stripping steam condensateand then passed into a fractionation train wherein the extract isseparated into its aromatic constituents. The nal lean solvent iswithdrawn from the rich solvent separation zone and returned to theextraction zone.

When the aromatic extract comprises a typical benzene, toluene, andxylene composition (a BTX composition), the benzene column of the BTXextract fractionation train will be provided with a side-cut withdrawalmeans for removal of the benzene product. This side-cut fraction isnormally withdrawn as a liquid stream from the fourth or fifth traybelow the top of the column in order to assure that it will besubstantially free of water and low boiling non-aromaics. The vaporwithdrawn overhead from the benzene column will contain the traces oflow boiling non-aromatic hydrocarbons as well as the traces of waterwhich are contained in the extract. The water which is contained in theoverhead of the benzene column stems from the stripping steam which isnormally utilized in stripping the aromatic extract out of the richsolvent in the prior rich solvent separation zone. The hydrocarbonfraction of the benzene column overhead is returned in-part to thebenzene column as reflux, and the water which is separated from thecondensed benzene overhead vapor is withdrawn from the process. Sincethe benzene overhead fraction contains low boiling nonaromaticconstituents, it is typically the art to withdraw a small slip stream ordrag stream from this receiver and pass the stream back to theextraction zone for separation of aromatics and non-aromatics. This dragstream assures that the top of the benzene column will not become loadedwith an accumulation of non-aromatic hydrocarbons which may eventuallycontaminate the benzene side-cut product stream.

The aromatic extraction process which has been broadly summarizedhereinabove is clearly set forth by D. B. Broughton et al., in U.S.Patent 3,361,664, wherein the solvent composition comprises sulfolaneand water. A typical aromatic extraction process wherein the solventcomposition comprises polyalkylene glycol and water is disclosed by I.H. Stephens in U.S. Patent 2,773,918. These patents and other publishedliterature clearly set forth typical processing steps and the operatingconditions for the aromatic extraction zone which produces anon-aromatic raflinate and an aromatic rich solvent, and for thesubsequent separation zone wherein the rich solvent may be separated toprovide a non-aromatic fraction, a high-purity aromatic extract, and afinal lean solvent.

As previously noted, the non-aromatic rainate which leaves theextraction zone will contain some solvent. The solvent may be present inthe rainate partly as a soluble constituent in low concentration andpartly as an entrained dispersion of free solvent phase due to theturbulence within the extraction zone. Because the typical solventcompositions which are utilized in aromatic extraction processing arewater soluble, it is the practice to extract the solvent which iscontained in. the non-aromatic rainate stream by contacting thisraffinate stream with an aqueous stream in a subsequent extractionmeans. The extraction of the solvent from the ranate with water may beundertaken in any sutiable liquid-liquid contacting means such as in atower containing suitable packing such as Berl saddles or Raschig rings,or in a tower containing suitable tray devices, or in a. rotating disccontactor (RDC).

The raflinate which then leaves the aqueous extraction zone, or waterwash zone, will be substantially free of this solvent composition butthe aqueous stream containing the recovered solvent will normallycontain some non-aromatic hydrocarbons. The non-aromatic hydrocarbonsare contained within the aqueous stream both as a soluble constituent ofthe aqueous stream and as a free hydrocarbon phase which is dispersedwithin the solventcontaining aqueous stream. This dispersion ofmicrodroplets of non-aromatic hydrocarbons is entrained within theaqueous phase because of the turbulence which is normally experiencedwithin the rainate water wash zone.

Thi-s solvent-containing aqueous stream is normally sent back to therich solvent separation zone to provide at least a part of the strippingsteam which is utilized in separating the aromatic extract from the richsolvent. As the aqueous stream is generated into stripping steam, thesolvent contained therein is thus recovered as a part of the nal leansolvent which remains when the extract has been stripped out of the richsolvent. However, since this aqueous stream contains a substantialportion of nonaromatic hydrocarbons, it is undesirable to utilize thisstream for direct generation of stripping steam since the non-aromaticranate contained therein would be vaporized in the stripping zone andthereby contaminate the resulting high purity aromatic extract. To avoidany contamination of the aromatic extract by non-aromatic constituentscontained within the aqueous stream, it is therefore, typically the artto pass this aqueous stream to a prior distillation column or Waterstill. The water still produces an overhead fraction comprising thenonaromatic raflnate constituents and water, and a bottoms fractioncomprising water and solvent having substantial freedom fromnon-aromatic contaminants. This bottoms fraction is then passed to theextract recovery column in order to provide at least a part of thestripping steam utilized therein, While the non-aromatic overheadfraction may be passed to the extraction zone as a part of thenonaromatic extractor reux.

SUMMARY OF INVENTION It is an object of the present invention to providean improved extraction process wherein high-purity aromatics areseparated from aromatic-containing feed stocks in a more economical andfacile manner. It is a particular object of this invention to provide Iamethod for the recovery of Water-soluble solvent from non-aromaticrafnate whereby the solvent may be recovered in an aqueous stream whichis reused as stripping steam without detrimentally contaminating thehigh purity aromatic extract with non-aromatics constituents. It is afurther particular object of the present invention to provide animproved extraction process for the recovery of high-purity aromaticswherein the solvent-containing aqueous stream from the rainate waterwash zone is used as a part of the stripping steam in the extractrecovery column by a method which provides for the elimination of theprior art water still.

Other objects and the advantages of the present invention will becomeapparent in the description which follows hereinbelow and which is madewith reference to the accompanying drawing. The attached drawing is aschematic ow diagram of a conventional aromatic extraction processwherein the prior art water still has been eliminated by the practice ofthe present invention.

It has been determined that the objects of this invention may beachieved by bringing the solvent-containing aqueous stream from therafnate water Iwash zone into contact with Ian aromatic hydrocarbonstream in a zone of high turbulence. Upon subsequent settling of theresulting mixture, substantially all of the non-aromatic constituentswhich were present in the solvent-containing aqueous stream will befound in the aromatic hydrocarbon phase. The resulting aqueous phasewill contain the solvent recovered from the non-aromatic raffinate sandsome aromatic hydrocarbons while having substantial freedom fromnon-aromatic hydrocarbons.

It is well known in the art that although water is a poor solvent forhydrocarbons, it has a high selectivity for -aromatic hydrocarbons. Forexample, at a temperature of 15.5 C. (60 F.), 100 grams of water willdissolve 0.168 gram of benzene, but only 0.014 gram of hexane.Similarly, at a temperature of 15.5 C. (60 F.), 100 grams of water willdissolve 0.040 gram of toluene, but only 0.005 gram of heptane (Seidell,A., Solubilities of Organic Compounds, vol. II, 3rd ed., 1941).

By the practice of the present invention, the aromatic hydrocarbonstream is admixed in a contacting zone with the solvent-containingaqueous stream under conditions sufficient to provide that substantiallyall non-aromatics will preferentially pass out of the aqueous phase andinto the hydrocarbon phase Iwhile the solvency of the water forhydrocarbons is utilized by selectively dissolving aromatichydrocarbons.

Therefore, in accordance with the practice of the present invention abroad embodiment comprises a process for the recovery of water solublesolvent from a tirst raffinate stream comprising non-aromatichydrocarbons produced in fan aromatic extraction process whichcomprises; contacting the first raflinate stream with a first aqueousstream in a first contacting zone under conditions suflicient to providea second rainate stream comprising non-aromatic hydrocarbons havingsubstantial freedom from the solvent, and a second aqueous streamcontaining solvent and non-aromatic hydrocarbons; contacting the secondaqueous stream with a first stream comprising aromatic hydrocarbons in asecond contacting zone under conditions suflicient to provide a secondaromatic stream containing nonaromatic hydrocarbons, and a third aqueousstream containing solvent and having substantial freedom fromnon-aromatic hydrocarbons; recovering the second raffinate stream fromthe first contacting zone and recovering the second aromatic stream fromthe second contacting zone; and recovering the solvent from the thirdaqueous stream.

A further embodiment of the present invention may be characterized asthe process disclosed in the broad embodiment immediately above, whereinthe third aqueous stream is returned to the aromatic extraction processto provide at least a part of the stripping steam required to removearomatic extract from the rich solvent composition.

A still further embodiment of the present invention may be characterizedas the process of the broad embodiment disclosed hereinabove, whereinthe first aromatic stream comprises a part of the aromatic extractproduced in the aromatic extraction process and the second aromaticstream is returned to the aromatic extraction process.

As has been noted hereinabove, the aromatic extract is normallyfractionated into its constituent aromatic hydrocarbons, and in thetypical BTX Aromatics Unit a benzene drag stream is removed from thebenzene column and sent back to the extraction zone. It is therefore,one preferred embodiment of the present invention to utilize thisbenzene drag stream as the first aromatic stream of the broad embodimentdisclosed above, and to return the benzene drag stream to the extractionzone after it has contacted the solvent-containing raffinate wash waterin the second contacting zone and has rendered the wash watersubstantially free of non-aromatic hydrocarbons.

This preferred embodiment and other embodiments may be more clearlyunderstood by now referring to the accompanying drawing in conjunctionwith the following disclosure of one specific example wherein thepresent invention is practiced.

DRAWING AND EXAMPLE In a specific operation illustrating the applicationof the inventive process, a depentanized catalytic reformate was rerunto remove the high boiling fraction, and then solvent extracted toproduce nitration grade benzene, nitration grade toluene, and a mixed C8aromatics stream. The rerun charge stock was passed to a primaryaromatics extraction zone, not shown in the accompanying drawing, at arate of 8730 b.p.s.d. or 1063.6 lb-mols/hr. wherein the feed wascontacted with a sulfolane solvent composition. A nonaromatic raffinatestream was produced at the rate of 4365 b.p.s.d. or 470.8 lbs.mols/hr.An aromaticcontaining rich solvent was produced at a rate sucient torecover 1191.9 lb.mols/hr. of hydrocarbon extract.

Referring now to the attached drawing, the raffinate stream leaves theprimary aromatic extraction zone and enters the process of the presentinvention at the rate of 470.8 mols/hr. via line 1. The raflinate inline 1 passes into a water wash extraction zone 2 which comprises an RDCextraction unit which is typical of the prior art. This raflinate streamcomprising 463.8 mols/hr. of hydrocarbon and 7.0 mols/hr. of sulfolanesolvent enters the RDC extractor 2 at 60 p.s.i.g. and 100 F. Theraflinate is Water washed therein by 621.9 mols/ hr. of water enteringthe top of RDC extractor 2 via line 3 at 100 F. The raffinate which hasnow been rendered substantially free of sulfolane solvent leaves RDCextractor 2 via line 4 at the rate of 463.73 mols/hr., at a temperatureof F., and at a pressure of 30 p.s.i.g. This raffinate stream is sent tofurther processing or to raffinate storage, not shown, at the rate of4320 b.p.s.d.

A rich aqueous phase leaves RDC extractor 2 via line 4S at the rate of628.97 mols/hr. and at a temperature of 100 F. This stream comprises621.9 mols/hr. of water, 7.0 mols/hr. of sulfolane solvent, and 0.07mol/hr. of non-aromatic rainate The non-aromatic hydrocarbon is presentin this rich water stream at a concentration which is greater than isnormally anticipated based solely upon solubility. This elevated levelof non-aromatic hydrocarbon contamination is due to the high turbulencewithin the RDC extractor 2 which results in the entrainment'of freehydrocarbon phase within the rich water stream of line 45. Thedisposition of this solvent-rich water stream will be further discussedhereinbelow.

The rich solvent stream from the primary aromatics extraction zoneenters the process of the present invention via line 5. This richsolvent stream contains 1191.9 mois/hr. of hydrocarbon extract andcomprises a solvent composition containing sulfolane and water. Thisrich solvent stream passes via line 5 at a temperature of 245 F. and apressure of 5 p.s.i.g. and enters solvent stripper column 6.

The solvent stripper is operated under conditions suffcient to removesubstantially all non-aromatic hydrocarbon contaminants from thesolvent-hydrocarbon solution. A stripper overhead vapor leaves solventstripper 6 via line 7 at the rate of 659.1 mols/hr., at a temperature of265 F., and at a pressure of 5 p.s.i.g. This vapor stream is cooled incondenser 8 to a temperature of 120 F. and then passed via line 9 into aphase separator 10 which is maintained at substantially atmosphericpressure. The vapor condensate is settled within separator 10 to providean aqueous phase and a hydrocarbon phase. The hydrocarbon phasecomprising non-aromatics is withdrawn via line 11 at the rate of 592.1mols/hr. and at a temperature of F. This hydrocarbon stream is typicallyreturned to the primary aromatics extraction zone in order to provide anon-aromatic reflux in the extractor vessel. An aqueous phase isseparated in separator 10 comprising a portion of the water in thesulfolane-water solvent composition which was vaporized from the solventcomposition which entered column 6. This aqueous phase is withdrawn fromseparator 10 via line 12 at the rate of 67.0 mols/hr. and is processedin a manner which will be set forth hereinbelow.

As the rich solvent liquid passes down solvent stripper column 6, itstemperature is elevated by rising hot hydrocarbon and water vapors in amanner sufficient to provide that substantially all non-aromatichydrocarbon constituents are removed from the liquid solvent phase. Ahot solvent liquid is withdrawn from the bottom of stripper column 6 vialine 13 at a temperature of 300 F. This solvent stream enters reboiler14 wherein the temperature is elevated to 350 F. A resulting hotreboiler vapor and liquid mixture leaves reboiler 14 via line 15 andreenters the bottom of stripper column y6 at l0 p.s.i.g.

A resulting stripper bottoms lfraction comprising rich solvent leavesthe bottom of the stripper column 6 via line 16 at a temperature of 350F. This rich solvent stream has been rendered substantially free ofnon-aromatic hydrocarbon contaminants by the operating conditions withinstripper column 6. The stripper bottoms fraction is now passed via line16 into an extract recovery column 17 at a temperature of 325 F. and apressure of 300 mm. Hg absolute. This rich solvent feed contains 599.8mols/hr. of substantially pure aromatic hydrocarbons and comprises asulfolane solvent composition containing water.

Extract recovery column 17 is operated in a manner sufficient to providethat substantially all aromatic hydrocarbons are separated from thesulfolane solvent composition. An overhead vapor stream leaves column 17via line 18 at a temperature of 180 F., and at a pressure of 300 mm. Hgabsolute. This hot vapor stream enters condenser 19 wherein it is cooledto 100 F. before passing into receiver 21 via line 20. The overheadvapor condensate is separated therein to provide an aromatic hydrocarbonphase and an aqueous phase which results from the stripping steamutilized within column 17. A first portion of the hydrocarbon phase iswithdrawn from receiver 21 via line 22 and is returned to extractrecovery column 17 as reflux. A second portion of the hydrocarbon phaseis withdrawn via line 23 and passed to the subsequent benzene column 32via line 23 at the rate of 602.8 mols/ hr. The aromatic extract passingvia line 23 comprises 599.8 mols/hr. of high-purity aromatichydrocarbon, and 3.0 mols/hr. of dissolved and entrained water. Theaqueous phase comprising stripping steam condensate is settled inreceiver 21 and withdrawn therefrom via line 3 at the rate of 621.9mols/hr. This aqueous stream is then passed to RDC extractor 2 via line3 as the raffinate water wash stream previously disclosed hereinabove.Because of a continual loss of water in the aromatic extract leavingreceiver 21 via line 23, a water make-up stream comprising clean steamcondensate is introduced into receiver 21 va line 24 at the rate of 3.0mols/ hr.

As the aromatic rich solvent passes down extract recovery column 17, itis contacted with stripping steam and hot aromatic vapors in a mannersufficient to strip substantially all hydrocarbons out of the solvent. Aresulting lean solvent leaves the bottom of recovery column 17 via line25 at a temperature of 325 F. A stream comprising stripping steam,hydrocarbon vapor, and sulfolane solvent enters line 25 via line 2-6from a source to be disclosed hereinbelow, and the resulting steam andsolvent stream passes into reboiler 27 wherein it is heated to 335 F.The resulting reboiler vapor and liquid mixture leaves reboiler 27 andenters the bottom of recovery column 17 via line Z8.

A net lean solvent leaves recovery column 17 via line 29 at atemperature of 325 F., and enters a steam generator 30 at .a pressure of150 p.s.i.g. wherein the stripping steam of line 26 is generated in amanner to be further disclosed hereinbelow. The lean solvent then leavesthe steam generator 30 via line 31 and is returned to the primaryaromatic extraction zone, not shown.

The high purity aromatic extract in line 23 is passed into benzenecolumn 32 at the rate of 602.8 mols/hr. This feed stream comprises 599.8mols/hr. of substantially pure aromatic hydrocarbon and 3.0 mols/hr. ofdissolved and entrained water, and enters column 32 at a temperature of225 F. and a pressure of 5 p.s.i.g. Benzene column 32 is operated underconditions sufficient to provide that substantially all the water andlow boiling non-aromatic contaminants will Abe removed as an overheadvapor while a substantially pure benzene product which meets nitrationgrade specifications will be produced as a side-cut fraction. In theinstant example, a hot benzene liquid is withdrawn from the fifth deckof benzene column 32 via line 33 at a temperature of 195 F. to provide anet benzene product of 158.6 mols/hr. Upon cooling to F. the net benzeneproduct is passed via line 33 to storage facilities, not shown, at arate of 960 b.p.s.'d.

The benzene overhead vapor is removed via line 34 at a temperature of F.and a pressure of 5 p.s.i.g. The vapor stream enters condenser 35wherein it is cooled to 100 F. before passing into receiver 37 via line36. A rst portion of the benzene overhead fraction is removed fromreceiver 37 via line 38 and returned to the top tray of column 32 asreflux. An aqueous phase is separated in receiver 37 and removedtherefrom via line 39 at the rate of 3.0 Inols/hr. and sent to adisposal system, not shown. In order to provide that non-aromatichydrocarbon contaminants will not accumulate within receiver 37 orwithin the top section of benzene column 32 and thereby contaminate thepure benzene withdrawn via line 33, a benzene drag stream is removedfrom receiver 37 via line 40. This drag stream comprises 8.3 mols/hr. ofbenzene and 0.008 mol/hr. of non-aromatic hydrocarbons and is processedin a manner which will be set forth hereinbelow.

The heavier aromatic constituents of the aromatic extract are renderedsubstantially free of benzene in the lower section of benzene column 32.A heavy aromatic stream comprising of C7-I- hydrocarbons is withdrawnfrom the bottom of benzene column 32 via line 41 and passed intoreboiler 42. This reboiler liquid stream is increased in temperaturefrom 270 F. to 280 F. therein, and returned to the bottom of the benzenecolumn 32 via line 43. A net aromatic fraction comprising C7| aromaticsis withdrawn from benzene column 32 via line 44 at the rate of 432.9mols/hr. and at a temperature of 270 F. This C74- fraction is furtherseparated to provide a nitration grade toluene product and a high-purityCg-laromatic product in a subsequent fractionation train, not shown.

The solvent-containing water stream of line 4S which is removed from thewater wash extractor 2 at the rate of 628.97 mols/ hr. is combined withthe water stream removed from stripper overhead receiver 10I via line 12at the rate of 67.0 mols/hr. The resulting aqueous stream continues vialine 45 at a temperature of 100 F. and at a rate of 695.97 mols/hr. Thistotal water stream contains 7.0 mols/hr. of sulfolane solvent which wasrecovered from the raffinate and 0.07 mol/hr. of nonaromatic raffinatewhich was dissolved and entrained in the raffinate wash water. The totalaqueous stream is augmente'd by the benzene drag stream withdrawn fromreceiver 37 at the rate of 8.308 mols/ hr. The benzene drag stream`containing 0.008 mol/hr. of non-aromatics enters line 45 via line 40,and the total stream of hydrocarbon and water is passed to in-line mixer46 via line 45 at a rate of 704.278 mols/hr.

The temperature of 100 F. and the degree of turbulence within in-linemixer 46 is sufficient to provide that the non-aromatic hydrocarboncontained in the rafinate wash water is preferentially passed into thehydrocarbon phase and dissolved in the benzene hydrocarbon.Simultaneously, the solvency of the aqueous phase for hydrocarbon issatisfied by selectively dissolving aromatic benzene in the aqueousphase in substitution for the displaced non-aromatic raflinate. Themixture of hydrocarbon and water leaves mixer 46 via line 47 and entersa phase separator or settler 48. A net benzene drag stream leavesseparator 48 via line 49 and is returned to the primary aromaticsextraction zone, not shown, at the rate of 48.5 b.p.s.d. This returnedbenzene stream comprising 7.95 mols/hr. of benzene and 0.076 mol/hr. ofnon-aromatics is returned to the extraction zone for the recovery of thebenzene and the rejection of the nonaromatic contaminants into theraffinate stream.

The water phase leaves settler 48 via line 50l at the rate of 696.252mols/hr. at a temperature of 100 F. This Water stream has now beenrendered substantially free of non-aromatic contaminants. The Waterstream in line 50 comprises 688.9 mols/hr. of water, 7.0 mols/hr. ofsulfolane, 0.35 mol/hr. of benzene, and only 0.002 mol/hr. ofnon-aromatic contaminants. The amount of hydrocarbon contained in thiswater stream is higher than anticipated from mere solubilityconsiderations, since the degree of phase separation in settler 48 isimperfect and some free hydrocarbon phase is entrained in the aqueousphase. In addition, the sulfolane solvent which is present in theaqueous phase increases the solubility of the benzene. The water streampasses via line 50 into steam generator 30 wherein the stream is heatedfrom 100 F. to 250 F. by the lean solvent stream, as has 'beenpreviously noted hereinabove. The resulting stripping steam leaves steamgenerator 30 via line 26 at the rate of 696.252 mols/hr. This strippingstream comprises 650.952 mols/hr. of saturated vapor and 45.3 mols/hr.of saturated liquid. The vapor comprises 650.6 mols/ hr. of steam, 0.35mol/hr. of benzene vapor, and 0.002 mol/hr. of nonaromatic vapor. Thesaturated liquid comprises 7.0 mols/ hr. of liquid sulfolane and 38.3mols/hr. of water. This hot liquid and vapor stream passes from streamgenerator 30 via line 26 into line 25 to provide the stripping streamrequired for recovery of the aromatic extract from the nal lean solventwithin column 17 as previously disclosed hereinabove.

PREFERRED EMBODIMENTS The effectiveness of the present invention may benoted by comparing the content of the non-aromatic hydrocarbon which wasoriginally contained in the solventcontaining rainate wash water leavingextractor 2, with the iinal non-aromatic in the aqueous stream which wasgenerated into the required stripping steam for the extract recoverycolumn 17. In the foregoing example, the nonaromatic hydrocarbon levelwas reduced from 0.07 mol/hr. to 0.002 mol/hr. or more comparatively,this reduction of non-aromatic contaminants was from a level of 111.2p.p.m. to 2.9 p.p.m. Thus, by the practice of the present invention thefinal aqueous stream of line 50 was rendered substantially free ofnon-aromatic contaminants, thereby eliminating the need for the priorart water still. Although this reduced non-aromatic hydrocarbon contentis passed overhead with the aromatic extract in the recovery column 17,this 2.9 p.p.m. of contaminants in the stripping steam only becomes 3.3p.p.m. of contaminants in the total extract which is withdrawn via line23. A1- though this 3.3 p.p.m. of non-aromatic becomes greatlyconcentrated in the benzene column, it does not appreciably affect thepurity of the benzene product withdrawn as the side-cut fraction. Thebulk of this non-aromatic contaminant which has been introduced with thearomatic extract feed, passes overhead with the benzene vapor and isultimately withdrawn as part of the benzene drag stream.

By the elimination of the water still in the process of the presentinvention both capital expense and operating expense are reduced. Thereboiler of the water still is replaced by the steam generator 30. Thecolumn of the water still and its other appurtenances are replaced bymixer 46 and separator 48 which are pieces of equipment requiring alower total capital expense than a fractionating column. It should benoted particularly that mixer 46 need not be an elaborate piece ofapparatus but may be any simple mixing device suitable for providing aturbulent contact between the aqueous phase and the aromatic hydrocarbonphase. A typically simple mixing apparatus suitable in this service maycomprise One or more mixing orifices.

Although the process of the example uses the benzene drag stream as thearomatic phase passed to the contacting zone, it must be realized thatthe scope of the present invention is not so limited. Any aromaticstream of high-purity is suitable Within the scope of the presentinvention and, in fact, an aromatic stream could be derived from somesource external to the aromatic extraction processing unit. However, itis preferable that some part of the aromatic extract which is producedin the process should be utilized, such as a slip stream withdrawn fromline 23, for example. However, since a benzene drag stream is normallywithdrawn in the typical commercial process, this is the most convenientsource for the aromatic stream passing to the inventive contacting zone.But it is also within the scope of the present invention to use atoluene stream in the contacting zone or to use a xylene stream forcontacting the aqueous phase. It must also be noted that where thearomatic extraction processing unit processes a hydrocarbon charge stockwhich does not contain benzene, the lowest boiling aromatic constituentof the extract will contain any nonaromatic contaminants contained inthe extract. Therefore, it is typical in the art, for example, whenfractionating extract comprising only toluene and xylenes to remove thewater contaminant and non-aromatic contaminants in the toluene overheadvapor and to remove the pure toluene product as a side-cut fraction in amanner similar to that which has been shown for the benzene column ofthe example. Thus, in such an aromatic extraction unit it is typicallynecessary to withdraw a toluene drag stream in order to avoidcontamination of the pure toluene sidecut product by an accumulation ofnon-aromatics in the top of the toluene column. In such an operation, itwould be preferable to use the toluene drag stream as the aromatichydrocarbon passed to the contacting zone.

While any high-purity aromatic mixture or any highpurity aromaticcompound is effective within the process of this invention, it ispreferable to use the lowest molecular weight aromatic constituent whichis available. This is because it is well known that solubility decreaseswith increasing molecular weight. Thus, in order to most effectivelydisplace the non-aromatic hydrocarbon out of the aqueous phase, thelowest molecular weight constituent of the aromatic extract shouldpreferably be used.

It must be realized that the operating conditions which have been givenin the foregoing example are specific to that example and should not beconstrued as a limitation upon the operation of the present invention.Those skilled in the art may readily ascertain those particularoperating conditions which may be required in order to achieve any givenseparation of non-aromatic raflinate and aromatic extract for any givencomposition of hydrocarbon charge stock. Broad operating conditions foroperation of the primary aromatics extraction zone and for the operationof the rich solvent separation zone may be found in the U.S. patentswhich have been cited hereinabove or in other well-known publications.Those skilled in the art similarly are able to select the operatingconditions which may be required in the benzene column and in thesubsequent aromatic fractionating columns utilized in the typicalfractionating train for the aromatic extract.

The operating conditions which may be required within the inventivecontacting zone comprising mixer 46 and separator 48 are also specificto the example. Any pressure could be imposed upon this contacting zoneprovided that the level of pressure is sufiicient to keep the two phasesin a liquid state under the temperature level which is utilized forcontacting the aromatic hydrocarbons with the aqueous phase. Thetemperature level should be established sufficient to selectivelydissolve the aromatic hydrocarbon into the aqueous phase whilepreferentially dissolving the non-aromatic hydrocarbons into thearomatic phase. It is well known that the separation between aromaticand non-aromatic hydrocarbons will be more selective at a relatively lowtemperature. Since the nonaromatic raffinate which leaves the aromaticextraction zone is typically at a temperature of from 200 F. to about300 F., the temperature of the contacting zone should not exceed thislevel. Indeed, it is preferable that the aromatic phase contact theaqueous at a temperature in the range of from about 60 F. to about 200F. and, preferably, the temperature selected will be in the neighborhoodof from about 80 F. to about 120 F. The turbulence which is experiencedin the contacting zone need not be defined with any specific limitationother than that it be sufficient to result in the intimate contacting ofthe aromatic and aqueous phases.

The amount of aromatic hydrocarbon which should be present in thecontacting zone should be as high as is possible frorn the standpoint ofequipment size and utility expense. The presence of a massive quantityof aromatic phase in comparison with the non-aromatic contaminants inthe aqueous phase will enhance the effectiveness of the presentinvention. By providing a more massive dilution of the non-aromatics inthe aromatic phase, the solubility of the non-aromatics in the aromaticphase is greatly promoted and the transfer of the non-aromatics from theaqueous phase to the aromatic phase becomes more highly preferential. Inthe example shown above, the ratio of the aromatic hydrocarbon to thenon-aromatic hydrocarbon in the contacting zone was 8.30 mols/hr. to0.0078 mol/ hr. or an effective ratio of about 1063 to one. It is Withinthe scope of this invention to operate the contacting zone with a ratioof aromatic hydrocarbons to non-aromatic hydrocarbons in the range offrom about 100 to one to about 10,000 to one or even higher.

Those skilled in the art can readily ascertain specific operatingconditions which may be required within the contacting zone for anyspecific application of this invention by utilizing the teachings whichhave been presented hereinabove.

From the foregoing disclosure, one broad embodiment of the presentinvention may now be summarized as a process for recovering aromatichydrocarbons from a charge stock containing non-aromatic hydrocarbonswhich comprises, contacting the charge stock with specified solvent inan extraction zone maintained under conditions sufficient to provide afirst raffinate stream comprising solvent-containing non-aromatichydrocarbons, and a rich solvent stream containing aromatichydrocarbons; passing the rich solvent stream from the extraction zoneinto a first separation zone under conditions sufficient to provide ahigh-purity aromatic stream, a first aqueous stream, and a lean solventstream substantially free of aromatic hydrocarbons; returning the leansolvent stream to the extraction zone to provide at least a part of thespecified solvent; contacting the first raffinate stream with the firstaqueous stream in a first contacting zone under conditions sufficient toprovide a second rainate stream comprising non-aromatic hydrocarbonshaving substantial freedom from specified solvent, and a second aqueousstream containing specified solvent and non-aromatic hydrocarbons;contacting the second aqueous stream with a first portion of thehigh-purity aromatic stream in a second contacting zone under conditionssufficient to provide an impure aromatic stream containing non-aromatichydrocarbons and a third aqueous stream `containing specified solventand having substantial freedom from non-aromatic hydrocarbons; passingthe impure aromatic stream into the extraction zone under conditionssutiicient to recover aromatics therefrom; passing the third aqueousstream into a heat exchanger means under conditions sufficient toprovide a fourth aqueous stream comprising specified solvent and steam;passing the fourth aqueous stream into the first separation zone toprovide at least a part of the stripping steam; and recovering thesecond raffinate stream and recovering a second portion of thehigh-purity aromatic stream.

The invention claimed is:

1. Process for recovery of water soluble solvent from a first raffinatestream comprising non-aromatic hydrocarbons produced in an aromaticextraction process which comprises:

(a) contacting said first rafiinate stream with a first aqueous streamin a first contacting zone under conditions suiiicient to provide asecond raffinate stream comprising non-aromatic hydrocarbons havingsubstantial freedom from solvent, and a second aqueous stream containingsolvent and non-aromatic hydrocarbons;

(b) contacting said second aqueous stream with a first aromatic streamcomprising high-purity aromatic hydrocarbons in a second contacting zoneunder conditions sufficient to provide a second aromatic streamcontaining non-aromatic hydrocarbons, and a third aqueous streamcontaining solvent and having substantial freedom from non-aromatichydrocarbons;

(c) recovering said second raffinate stream from first contacting zone,and said second aromatic stream from said second contacting zone; and,

(d) recovering said solvent from said third aqueous stream.

2. Process of claim 1 wherein said third aqueous stream is passed tosaid aromatic extraction process.

3. Process of claim 1 wherein said first aromatic stream comprises apart of an aromatic extract produced in said aromatic extraction processand said second aromatic stream is lpassed to said aromatic extractionprocess.

4. Process of claim 1 wherein said solvent comprises a sulfolane-typechemical of the general formula:

Rg-CH CII-R4 Rz-CH-CH-Ra wherein R1, R2, R3, and R4 are independentlyselected from the group comprising a hydrogen atom, an alkyl grouphaving from one to ten carbon atoms, an arylalkyl radical having fromone to twelve carbon atoms, and an alkoxy radical having from one toeight carbon atoms.

5. Process of claim 1 wherein said solvent comprises at least onepolyalkylene glycol.

6. Process for recovery of aromatic hydrocarbons from a. charge stockcontaining non-aromatic hydrocarbons which comprises:

(a) contacting said charge stock with specified solvent in an extractionzone maintained under conditions sufficient to provide a first raffinatestream comprising solvent-containing non-aromatic hydrocarbons, and arich solvent stream containing aromatic hydrocarbons;

(b) passing said rich solvent stream from said extraction zone into afirst separation zone;

(c) passing stripping steam into said first separation zone underconditions sufficient to provide a highpurity aromatic stream, a firstaqueous stream, and a lean solvent stream substantially free of aromatichydrocarbons;

(d) returning said lean solvent stream to the extraction zone to provideat least a part of said specified solvent;

(e) contacting said first raffinate stream with said first aqueousstream in a first contacting zone under conditions suflicient to providea second raffinate stream comprising non-aromatic hydrocarbons havingsubstantial freedom from specified solvent, and a second aqueous streamcontaining specified solvent and nonaromatic hydrocarbons;

(f) contacting said second aqueous stream with a first portion of saidhigh-purity aromatic stream in a second contacting zone under conditionssufficient to provide an impure aromatic stream containing nonarornatichydrocarbons and a third aqueous stream containing specified solvent andhaving substantial freedom from non-aromatic hydrocarbons;

g) passing said impure aromatic stream into said eX- traction zone underconditions sufficient to recover aromatic therefrom; g

(h) passing said third aqueous stream into said first separation zone ina manner sufficient to provide at s least a part of said strippingsteam; and,

(i) recovering said second rafiinate stream and recovering a secondportion of said high-purity aromatic stream.

7. Process of claim 6 wherein said high-purity aromatic stream isseparated at least in part into its aromatic molecular constituents, andsaid first portion of said highpurity aromatic stream comprises aportion of one of said constituents.

8. Process of claim 7 wherein said first portion of said high-purityaromatic stream comprises a portion of the lowest molecular weightaromatic constituent.

9. Process of claim 6 wherein said specified solvent comprises asulfolane-type chemical of the general formula:

Ri-CH \CH-Ra Rz- H- H-R:

wherein R1, R2, R3, and R4 are independently selected from the groupcomprising a hydrogen atom, an alkyl group having from one to ten carbonatoms, an arylalkyl radical having from one to twelve carbon atoms, andan alkoxy radical having from one to eight carbon atoms.

10. Process of claim 6 wherein said specified solvent comprises at leastone polyalkylene glycol.

11. Process of claim 6 wherein said third aqueous stream is passed intoa heat exchanger means under conditions sufficient to provide a fourthaqueous stream comprising specified solvent and steam, and said fourthaqueous stream is passed into said first separation zone in a mannersufi'icient to provide at least a part of said stripping steam.

References Cited UNITED STATES PATENTS 2,936,283 5/1960 Hutchings208-321 3,179,708 4/1965 Penisten 208-321 3,308,059 3/1967 Deal 208-3213,338,824 8/1967 Oliver 208-321 HERBERT LEVINE, Primary Examiner U.S.Cl. X.R. 208-325, 333

